Osteoarthritis (OA) is one of the most prevalent joint disorders that often leads to severe disability. Despite the incidence, relatively little is known regarding the pathogenesis of OA although both cellular and biomechanical mechanisms are implicated. Knee OA commonly develops following anterior cruciate ligament (ACL) injury despite surgical reconstruction, particularly in females. The high incidence of ACL injuries in female athletes, combined with the high risk for future joint damage, motivates investigation of mechanisms underlying the development of knee OA in this population. Quadriceps strength deficits are common following ACL reconstruction and persist long after structured rehabilitation and after patients have returned to high-level activities. Quadriceps muscle weakness is associated with altered joint mechanics that may propagate articular cartilage damage. Little information is known regarding the impact of impaired quadriceps strength on joint mechanics during high-level activities or on longitudinal clinical outcomes. It is suspected that if athletes with ACL reconstruction return to high-level activities and expose their knee to substantial forces without normalized quadriceps strength and joint mechanics, the conditions for articular cartilage damage may be initiated and propagated. Appropriate guidelines for safe return to high-level sport activities following ACL reconstruction are not known. Female athletes are a unique subpopulation within those with ACL injury, and several unique characteristics suggest that they may respond to ACL injury and ACL reconstruction differently than males. The Specific Aims of this proposal involve careful, longitudinal examination of the impact of residual quadriceps strength deficits on knee joint mechanics and on functional outcomes in female athletes with ACL reconstruction. This study represents a comprehensive investigation of female athletes following ACL reconstruction and will serve as a necessary step in establishing safe and appropriate rehabilitation guidelines. Knee OA is a progressive joint disease that often leads to severe pain and disability. Individuals with ACL injury have substantial risk of developing knee OA despite ACL reconstruction. This investigation will provide valuable information regarding potential mechanisms involved in the initiation of arthritic joint changes following ACL injury and reconstruction. Information from this study will serve as springboard for future research of specific interventions that may reduce the risk of arthritic joint changes following ACL injury in female athletes.